This disclosure relates generally to the seismic surveillance and, in particular, to methods for acquiring seismic and other signals that are representative of the subsurface for purposes of seismic exploration and/or surveillance.
A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. Conventionally each trace records one seismic source activation, so there is one trace for each live source location-receiver activation. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
In seismic acquisition a marine source array, often an array of air guns, is composed of many single units that are towed behind a vessel that travels over the survey area. These units (e.g., air guns, water guns, sparkers, boomers, chip systems, water sirens, etc.) are typically hung in a line under a sausage buoy to allow them to be towed in a streamlined fashion. It is typical in deep water seismic surveying to use 6 to 15 guns under a single buoy. This configuration of seismic sources is conventionally referred to as a sub-array. A conventional seismic source array is comprised of one or more sub-arrays, all of which are all conventionally activated simultaneously and thereafter treated as thought it were a single source.
Of recent interest is the use of so-called “Popcorn” survey techniques (as described more fully hereinafter) in marine seismic surveys. However, for all of the promise these sorts of surveys might hold, designing such surveys involves considerations that are different from those typically encountered in a conventional seismic survey.
It should be noted and remembered that the description which follows, together with the accompanying drawings, should not be construed as limiting the claims to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the disclosure pertains will be able to devise other forms of this disclosure within the ambit of the appended claims.